Forensic Engineering 7th Congress Performance of the Built Environment November 15 – 18, 2015 Hilton Miami Downtown Hotel Miami, FL A New Structural and Metallurgical Review of the Great Boston Molasses Tank Failure of 1919 Ronald A. Mayville1 Simpson Gumpertz & Heger Inc., 41 Seyon Street, Waltham, MA, 02453; PH (781) 907-9414; FAX (781) 907-9000; e-mail: [email protected] 1 ABSTRACT The Boston molasses tank failure is a famous incident in which 21 people lost their lives. The cause of the failure was never definitively determined even though the court auditor was presented with over 20,000 pages of conflicting testimony by prominent engineering experts from institutions like MIT and Harvard. The author has examined evidence presented in the case as well as private failure analysis reports by experts of the day in light of our current understanding of structural design, analysis methods, and metallurgical effects. There were no applicable structural codes when the tank was designed, but the practice for designing riveted structures was documented in well-known text books. The author has conducted finite element analysis to calculate the stress and strain conditions that existed just prior to failure as well as to simulate the tank collapse and flow of molasses against adjacent structures, many of which were destroyed. The case is also examined with respect to the methods we currently use to protect against brittle fracture, including the use of toughness tests and the effects of steel chemistry. The work includes discussion on the differences between the structural steel used to fabricate the tank and what we use today, including an interesting reference to the Titanic failure. INTRODUCTION Great structural failures have occurred throughout history and it is human nature to try to understand why they occurred for both the betterment of society and to determine who is at fault. The great Boston molasses failure of 1919 is a particularly interesting case because of the number of fatalities (21) and the curious manner in which people perished. Nearly two and a half million gallons of thick liquid suddenly and without resistance flowed as a great wave into the streets of Boston leveling buildings like a tsunami and nearly collapsing the adjacent elevated railway. For decades after the accident, residents claimed they could smell of molasses on hot days. An account of the social aspects of the tank failure has been provided by Puleo [1]. The failure led to a four-year long hearing that produced 25,000 pages of testimony supported by many experts from leading institutions of the time like MIT and Harvard. Numerous tests were conducted by the experts including the fabrication of a sub-scale tank in which a bomb was exploded in an effort to demonstrate the similarities between the outcome of that test and the actual tank failure. This paper presents the results of a fresh look at the tank failure using some of the forensic tools we have available to us today that they did not have then. In particular, we use both finite element analysis and fracture mechanics to evaluate the conditions under which the failure occurred. The author has had a long interest in the tank failure stemming from an early fracture mechanics class and fueled by residence within a few miles of the original tank site. Figure 1. The molasses tank in the background with the elevated railway to the left. The buildings shown were destroyed in the accident. (Photo: Courtesy of Bill Noonan, Boston Fire Department Archives.) The tank ruptured at about noon on January 15, 1919, destroying the buildings shown in Figure 1 and damaging the elevated railway, Figure 2. The temperature at the time of failure, and for several hours prior to the failure, was about 40°F. There was no snow on the tank and the reported wind speed was about 15 mph. The tank had been filled to a level of 2.3 million gallons just two days prior to the failure, but had previously experienced about 30 substantial changes in the fill level. The tank broke into several pieces including a four ton piece of the bottom ring 1 that included half of the manway and part of the tank bottom. This piece was found about 200 ft from the original tank location (in the foreground of Figure 1) and at least one expert testified its position was the result of a parabolic trajectory resulting from an explosion within the tank. Figure 3. Damage to the adjacent elevated railway from the molasses tank failure. (Photo: Courtesy of Bill Noonan, Boston Fire Department Archives.) TANK DESCRIPTION The molasses tank was owned by U.S. Industrial Alcohol (USIA) who converted the molasses into alcohol for the production of munitions to support the First World War. The tank was designed and constructed by the Hammond Iron Works of Warren, Pennsylvania. Most of the information about the tank comes from the testimony of experts during the hearing. It was 90ft in diameter and 50ft tall and consisted of seven courses or rings of open hearth steel plate; the bottom ring had a thickness of 0.67 in. Horizontal lap joints had a single row of rivets. The bottom ring utilized a butt joint with inner and outer cover plates and three rows of rivets on each side. The vertical joints of the upper rings used fewer rivets. There were no industrial design codes for tanks at that time, but the City of Boston building code placed limits on the maximum stress in ‘structures’ that depended on the material and the potential failure mode. Figure 4 lists the values applicable to the steel of the tank. Figure 4. An excerpt from the Boston Building Law applicable in 1919 The tank included a single manway located in the first ring. It was riveted to the tank and included a flange, but only on the exterior. In addition, the lap joint of this flange was located at the top of the manway, which as our finite element analysis has shown (see below) coincided with the maximum stress around the manway cut out. Figure 5 shows a photograph of a replica of the tank manway that was fabricated by one of the experts, Albert Ladd Colby, for a large-scale test at Lehigh University. (The machine in which it was tested in 1920 still stands at the university.) Rivet holes were made in the field at the site of the tank construction by punching and the punched holes were not subsequently reamed. The consequences of such a fabrication method were studied extensively even in that time, primarily because of the associated degradation in strength that occurred in boiler construction. Punching alone could either reduce the ductility of the material adjacent to the hole or cause cracks or both. Figure 5. Replica of the molasses tank manway construction (Photo: Courtesy of Lehigh University) FRACTURE AND MATERIAL EVIDENCE The pieces of the failed tank were analyzed and tested extensively by the various experts and results were reported in court testimony. Notable points include: The tank broke into several large pieces, with one large section pushed West towards the elevated railway and others pushed North and East of the original location. A four ton piece that included part of the manway and a pie-shaped section of the tank bottom was found about 200 ft South in the playground shown in Figure 1. Vertically oriented fractures occurred both above the manway, at the 12 o’clock position, and below the manway, at the 6 o’clock position. The corresponding fracture surfaces showed herringbone features that pointed to the manway as the fracture origin for both the upper and lower fractures. Tensile properties showed the tank steel to be of good quality of the day: yield strength, about 35ksi, tensile strength, 56 ksi, elongations in 8 in. over 20%, and reductions in area exceeding 50%. Metallographic sections taken immediately adjacent to several of the fractures revealed Neumann bands (what we now refer to as deformation twins.) The last point is a particularly interesting one because several experts for the defense (USIA) used such evidence to declare that an explosion within the tank was responsible for the failure. Considerable research had been conducted as part of military developments to support this statement. Nevertheless, we know now that Neumann bands can be produced by the very rapid crack propagation that occurs in fracture of brittle steels [2]. STRESS ANALYSIS At the time the tank was designed, stress analysis was conducted only by hand. Design formulas were available from text books for riveted joints but the concept of stress concentration factors was apparently not used in analysis of structures. The maximum nominal hoop stress in the tank, considering the head pressure of the molasses at the time of failure but without consideration of corner or lap joint reinforcement, was 28.7 ksi. The calculated bearing and rivet shear stresses for the vertical diamond pattern butt joint in the first ring were 15.4 and 32.4, respectively [3]. Each of these three stress quantities exceeds the stress limits of the Boston Building Code (Figure 4) by a large amount: 55 to 80% depending on the type of stress. It is a mystery to the author how the designers could make such a large mistake. Perhaps the they thought the tank would be filled with water, which is 40% lighter than molasses. We have now conducted finite element analysis of the region of rivet holes adjacent to the manhole using the computer program Abaqus [4]. The stress-strain curve for the tank steel was estimated from the tensile properties and we did not consider the possible effect of work hardening from the riveting process. The results show that the most highly stressed area, which is the rivet hole directly above the manhole, likely experienced plastic deformation; Figure 6. Figure 6. Contours of calculated plastic strain at the rivet holes of the molasses tank corresponding to the molasses level at the time of failure. Fracture mechanics can be used to assess the severity of a punching-induced crack that may have existed at the edge of one of the manway rivet holes. Such an analysis is complicated by the nonuniform stress distribution in the vicinity of the manway and the fact that there was plastic deformation, but one can use linear elastic fracture mechanics to approximately assess the risk of crack growth. The stress intensity for a small crack at the edge of a hole under uniform tension is given by the equation [5]: 𝐾𝐼 = 𝐹𝜎√𝜋𝑎 where F = 3.4. We do not know the fracture toughness of the molasses tank steel, but we can estimate it from the data for a comparable steel of the time. The Titanic was fabricated from such steel and after discovery of this structure in 1986 material was recovered and tested [6]. We estimate the toughness at 40°F, the temperature at the time of the accident, to be 30 ksi√in. Using this value with a nominal stress of 25 ksi gives a critical crack size of 0.05 in., about 5% of the hole diameter. Again, this is not an accurate calculation because of the presence of plastic deformation and because the thickness of the plate is less than required to provide plain strain conditions. We have also conducted finite element analysis to simulate the actual failure of the tank. The approach uses the Eulerian methods of Abaqus and includes the fluid properties of the molasses and the deformable properties of the tank. The current model simplifies the fracture as having occurred in a few specific locations. Figure 7 shows an image at approximately two seconds after initial failure and even shows how the four ton, pie-shaped section could be moved by the molasses alone without the aid of an explosion. Figure 7. Dynamic simulation result of the molasses tank failure two seconds after failure initiation. DISCUSSION The initial, public judgment of the molasses tank failure was that it arose from an explosion. The defense in the subsequent hearing used the evidence of tank pieces having been moved great distances with great force, the physical similarity of the fracture patterns and the metallurgical evidence of Neumann bands in an effort to prove that there had been an explosion. The plaintiffs could easily show that the tank was underdesigned and were able to find alternate explanations for the evidence on which the defense relied. The auditor ruled in favor of the plaintiffs and awarded them $300,000 (about $4 million in 2014 dollars.) Our results support the conclusion of the auditor. We have shown that the stresses and strains around the rivet hole at the top of the manway were severe. Our estimate of the stress intensity factor for a crack at this location indicates that a pre-existing crack from the rivet hole punching process could have been sharpened by the 30 significant cycles of tank filling eventually leading to the initiation of rapid fracture. Our research continues with one of the main mysteries being how a reputable firm like the Hammond Iron Works could have made such a substantial design error. ACKNOWLEDGMENTS The author would like to acknowledge the assistance in searching the literature of Mr. Albert Saul and Ms. Joan Cunningham, both of Simpson Gumpertz & Heger Inc., and of Mr. Brian Harkins of the Social Law Library, and of Dr. Ozgur Yapar, of Dassault Systems for assistance in conducting the dynamic finite element analysis. REFERENCES 1. Puleo, Stephen, Dark Tide (Boston; Beacon Press)2003 2. Iung, T., Di Fant, M., and Pineau, A., Dynamic Fracture and Crack Arrest Behaviour of a Pipeline Steel Investigate with a New Specimen Geometry: The Ring Test,” in ECF9, Reliability and Structural Integrity of Advanced Materials (1992) 3. Mayville, R.A., “The Great Boston Molasses Tank Failure of 1919,” Civil + Structural Engineer (Sept. 2014) 4. Abaqus, Dassault Systems 5. Barsom, J.M. and Rolfe, S.T., Fracture and Fatigue Control of Structures, 2nd Edition, Prentice-Hall, NJ, 1987. 6. Foecke, T.,”Metallurgy of the RMS Titanic,” National Institute of Standards and Technology, Report NIST-IR 6118 (1998)18 pages.
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